1. Field of the Invention
The present invention is directed to an induction motor control system, and more particularly to an improved control system for controlling a variable speed induction motor on a constant volt/frequency basis.
2. Description of the Prior Art
Control systems for a variable speed induction motor are known in the art to include a vector control, slip-frequency control, and a constant volt/frequency control. These control systems utilize an inverter supplying a variable frequency AC voltage to the induction motor but differ in a manner of controlling the inverter. The constant volt/frequency control is devised to keep a motor flux constant by regulating the output voltage of the inverter in relation to a frequency in order to drive the motor efficiently, and therefor has been widely utilized among the other control systems for general application purposes by the reason that it requires no additional tachogenerataor essential in the slip-frequency control or no complicated operations inherent to the vector control.
However, at low frequency, the constant volt/frequency control technique suffers from a considerable torque drop due to a significant stator resistive drop, i.e., the reduction of the motor flux. That is, with a constant volt/frequency control it is practically impossible to regulate the torque in the low frequency range and therefore to handle a load which requires a great starting torque. To overcome the problem, the prior art has proposed a scheme of compensating for the stator resistive drop in order to obtain a constant induced air gap voltage per frequency rather than a constant inverter output voltage per frequency in the low frequency range. This scheme utilizes a predetermined volt/frequency relation or volt/frequency (V/F) curve arranged to compensate for the significant stator resistive drop at the low frequency. Such V/F curve is assigned to and effective for a specific load requirement, therefore the scheme further requires to use a number of V/F curves for different power or load requirements so that a suitable V/F curve can be selected in accordance with the torque pattern or load requirement already known to an intended work. As described, such prior art scheme is found to be effective only for a limited application where the torque pattern of a load is known, thus failing to provide a general purpose motor control system. In addition, the above prior scheme is found inconvenient in that the selection of V/F curves have to be done by an user.
To eliminate the above insufficiency, Japanese Patent Publication (KOKAI) No. 61-81197 has suggested an automatic selection of V/F curve in response to the load imposed on the motor. This patent teaches to derive the amplitude of the motor current at an angle of 90.degree. of the inverter output voltage and to utilize such amplitude of the motor current as an index of the load imposed in controlling the inverter. That is, the system monitors the amount of the motor current at a particular angle to indicative of the instantaneous load and selects a suitable V/F curve prepared for such load in a proportional control basis. Although the system enables an automatic selection of V/F curve or motor torque regulation with varying loads, the above proportional control requires a rather complicated analog circuit which is hindrance to reducing the manufacturing cost of the control system. In addition, the control system is found not to operate reliably at the low frequency. The unreliability comes from that the above index of the load becomes no longer reflect a correct load as the stator resistance will act again to disorder the phase relation between the motor current and the inverter output voltage to a serious extent at the low frequency. For instance, under no load condition, the phase angle between the motor current and the inverter output voltage becomes considerably less than 90.degree. due to the stator resistance remarkable at the low frequency, while the phase angle is kept about at slightly less than 90.degree. at the high frequency. Such serious variation of the phase angle at the low frequency will eventually results in the variation in the detected amount of the motor control and therefore leads to an unreliable or erroneous motor control. That is, even under no load condition, the system will detect a significant amplitude of the motor current at 90.degree. angle of the inverter output voltage and acknowledge a correspondingly amount of load being imposed, thereby controlling the inverter based upon such false detection to increase the output voltage in an attempt to follow the false load. The result is that the motor is overheated or driven with unacceptable high noise level.